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The present invention relates to multiple sealing systems for composite risers and methods of preventing interior fluid leakage to the outside of composite risers. More particularly, the present invention relates to an elastomer provided in single or multiple circumferential grooves on the outside of a metal to composite interface of a liner assembly for the composite riser. The elastomer in the grooves bonds with an outer elastomeric ply to provide elastomeric seals which, in combination with a seal between the metal to composite interface and a liner of the liner assembly, provides a multiple sealing system for the composite riser to prevent leakage of interior fluids.
As exploration and production of oil and gas move into deeper water, weight, cost and reliability of water-depth sensitive systems such as risers become increasingly important. The term riser generally describes the different types of discrete pipes that extend from the seabed toward the surface of the water. These include components such as drilling risers, production risers, workover risers, catenary risers, production tubing, production risers, choke and kill lines and mud return lines. Risers can be constructed of metal and, more particularly, of steel. More recently, composite risers are being considered.
The advantages that composites offer to deepwater risers are high specific strength and stiffness, lightweightedness, corrosion resistance, high thermal insulation, high damping and excellent fatigue performance. Capitalizing on these and other advantages for composite riser applications can result in lower system cost and higher reliability for deepwater developments. Efforts have been devoted during the recent years to assess the full potential of composite materials for deepwater riser applications. The cost savings and enabling capability of composite risers for deepwater drilling and production operations are particularly appealing.
Conventional composite risers are constructed of an outer composite material and an inner liner assembly. More particularly, in a conventional composite riser, a thin tubular metal or elastomeric liner is coaxially secured to the metal connections to form the liner assembly. An elastomeric shear ply is provided along the outer surface of the liner assembly, followed with a composite overwrap reinforcement to form the composite riser. The composite riser is heated to cure the elastomeric shear ply and the composite overwrap. An external elastomeric jacket and a layer of composite overwrap are provided over the composite assembly and thermally cured for external damage and impact protection to the composite riser. The liner assembly is necessary to prevent leakage due to the inherent cracking characteristics of the composite material. The matrix in the composite will develop micro cracks at pressures lower than those at which the composite fibers will fail. The matrix micro cracking is due to the thermal stresses induced by the curing cycle and the mechanical stresses induced during the shop acceptance pressure test of the composite riser during the manufacturing process. Thus, liner assemblies are essential in ensuring fluid tightness of composite risers to prevent leakage under the conditions of matrix cracking which is inevitable.
The integrity of the composite riser, particularly at the interface between the composite overwrap and the metal connector of the liner assembly, presents a reliability issue for composite risers. Composite risers with elastomeric liners have a seal at the termination between the metal connector and elastomeric liner which is formed by the bonding of the elastomeric material of the liner and an elastomeric material which is provided on the tip of the metal termination. The reliability of the sealing system is questionable, particularly given that environmental degradation occurs to the elastomers by the production fluids.
While elastomeric liners are acceptable for production composite risers, they are ill suited for use in composite drilling or workover risers. The likely possibility of damage to elastomeric liners by mechanical tools which are required for drilling and workover operations make the elastomeric liners undesirable for these types of operations. Thus, metal liners for composite drilling and workover risers are being considered. Metal liners also have applications as composite production risers as the metal offers better long term resistance to the production fluids than elastomers. In a conventional composite riser having a metal liner, the metal liner is welded directly to the metal connector at a section called the metal to composite interface (MCI). Alternatively, the metal liner is coaxially secured to the MCI through the use of a transition ring. The transition ring is secured at one end to the MCI and is welded at the other end to the metal liner. The ring can serve as a transition between the material of the liner and that of the MCI when different grade materials are required. For example, a liner and transition ring can be constructed of titanium, while steel can be used for the MCI. The integrity of the composite riser is, generally, dominated by the fatigue resistance of the weld between the liner and the MCI. In addition, the seal between the transition ring and the MCI is critical to the fluid tightness of the composite riser.
The present invention provides multiple sealing systems for composite risers, and methods of preventing interior fluid leakage to the outside of composite risers. The invention is applicable to composite risers having metal liners which are welded or mechanically secured to the metal to composite interface (MCI) directly or to a transition ring coupled to the MCI, and to composite risers having elastomeric liners.
One or more elastomeric seals are provided between the MCI and an elastomeric shear ply provided on the outside of the liner assembly of the composite riser. In metal lined composite risers, the elastomeric seals, in combination with the weld between the liner and MCI, or the mechanical seal between the MCI and transition ring, provide a multiple sealing system between the MCI and the metal liner to prevent leakage of interior fluids to the outside of the composite riser. In the event that the integrity of the liner welds or the mechanical seal are compromised, the elastomeric seals would prevent leakage of internal fluids. In composite risers having elastomeric liners, the elastomeric seals created between the MCI and elastomeric shear ply improve the reliability of the sealing system between the metal connector and the elastomeric liner.
The elastomerics seals comprise one or more grooves which are provided along the outer tubular surface of the MCI proximate its inboard end. An elastomer in an uncured state is injected into the elastomer grooves of the MCI. The elastomer bonds with the elastomeric shear ply provided on the outside of the liner assembly to form the elastomeric seals, as discussed in further detail below.
The elastomeric shear ply in an uncured state is applied over the entire length of the outer surface of the liner assembly, including over the MCI, the elastomer in the elastomer groove, the liner, and in specific embodiments having a metal liner assembly, over the transition ring. A composite overwrap is wound over the elastomeric shear ply to form a composite riser which is heated to cure the composite material, the elastomeric shear ply and the elastomer in the elastomer grooves. The heat bonds the elastomer in the elastomer groove with the elastomeric shear ply along their interface to form the elastomeric seals.
In a composite riser having a metal liner, the single or multiple elastomeric seals work in combination with a conventional mechanical seal between the metal liner and the MCI to provide a multiple sealing system for the composite metal lined riser. In a composite riser having a metal liner welded to the MCI, the mechanical seal is a weld.
In a composite riser having a metal liner secured to a transition ring which is coupled to the MCI, the mechanical seal comprises conforming inner and outer grooves of the MCI and transition ring, respectively, which engage to provide the seal. In this embodiment, a plurality of inner grooves is circumferentially provided along the inner surface of the MCI. Each inner groove is a mechanical interlock joint, such as for example, a Talon connector, which is used for steel tubular applications to provide a metal to metal seal. The conforming grooves of the transition ring comprise outer grooves which engage with the inner grooves of the MCI when the transition ring is fitted into the inboard end of the MCI. The conforming grooves engage to form the mechanical seal between the transition ring and the MCI and the metal liner is secured to the transition ring at its end opposite the MCI.
In a composite riser having an elastomeric liner, the single or multiple elastomeric seals between the elastomer in the outer grooves of the MCI and the elastomeric shear ply improve the reliability of the conventional sealing system provided at the termination between the metal connector and the elastomeric liner.